Hydraulic control system



NOV. 12, 1940. LAWRENCE 2,221,313

HYDRAULIC CONTROL SYSTEM Filed March 24, 1957 5 Sheets-Sheet l Nov. 12, 1940. WR N E- 2,221,313

HYDRAULIC CONTROL SYSTEM Filed March 24; 1957 5 Sheets-Sheet 2 .Z'nl enzar Han/are Flea/fence NOV. 12, 1940. wR c 2,221,313

HYDRAULIC CONTROL SYSTEM Filed March 24, 1957 5 Sheets-Sheet s Nov. 12, 1940.

HYDRAULIC CONTROL SYSTEM I Filed March 24, 1937 5 Sheets-Sheet 4 H. F. LAWRENCE 2,221,313 I Nov. 12, 1940. F, L W N E 2,221,313

HYDRAULI C CONTROL SYS TEM Filed March 24, 1937 5 Sheets-Sheet 5 wax-a [1a Mrewce 5.422s .ay g

' along line 66 of Fig. 2.

Fig. '7 is a sectional view taken substantially.

Patented Nov. 12, 1940 HYDRAULIC CONTROL SYSTEM Howard F. Lawrence, Philadelphia, Pa., assignor to American Engineering Company, Philadelphia, Par, a corporation of Pennsylvania Application March 24.1931, Serial No. 132,689

'7 Claims. This invention relates to hydraulic control systems, and more particularly to systems employedto reverse the direction of discharge of pumps when the pressure developed thereby exceeds a predetermined value. l

' One object of the present invention is to provide a generally improved andsimplified control system of the stated character which is fully automatic in operation.

Another'object is to provide a system of the stated character wherein the pump controls are positively actuated to effect a reversal of the discharge ther'eof.

A further and more specific object isto provide in a fluid system including a high pressure piunp for actuating feeding rams, a control device which automatically and positively'eflects reversal of the discharge of the pump when the rams reach their limit of travel in either direction. 1

Other and further objects-will become apparent as the description of the invention progresses. i

Of the drawings:

Fig. 1 is a plan View partly in section and diagrammatic in part, showing a general assembly of the hydraulic control system comprising the present invention.

Fig. 1 is a partial schematic view partly in section and with \certain parts removed to show the structure ofthe entire assembly constituting the present invention.

Fig. 2 is a side view partly in section, of the fluid motor for actuating of the pump controls and the valves controlling the operation thereof.

Fig. 3 is an end view of the mechanisms shown in Fig. 2.

Fig. '4 is a sectional view taken substantially along line 4-4 of Fig. 3.

Fig. 5 is a sectional view along line 55 of Fig.2.

Fig. 6 is asectional view taken substantially along line 1-1 of Fig. 2.

Fig. 8 is a diagrammatic view partly in section of an alternative construction, and

Fig. 9 is a sectional view taken substantially along line 9-9 of Fig. 8.

Referring to the drawings, the numeral l' indicates a variable stroke, reversible discharge pump connected in the-present instance by main pipes 2 and 3 and branch pipes 5 and-4 to the opposite ends of a series of cylinders 6. A piston taken substantially I operates in each cylinder 6, and isconnected in any suitable manner in. the present instance to a ram which feeds materials from hoppers 8 to any desired receiving means (not shown). .Pump may be of the general type shown and described in United States Patent No, 1,077,979, and the drive shaft 9 thereof is connected by any suitable coupling III to the armature shaftll of an electric motor l2. Pump and motor l2 arerigidly secured to a makeup or supply tank l3, from 10 which makeup'iluid maybe drawn into the system throughpipes l4 and I5 connected respectively to main pipes-2 and 3. i

The sliding blocks l8 of pump I which, as is well known in the art, control the quantity, as well as the direction of discharge of the pump,- are actuated by means of a fluid motor l9. Fluid motor l9 comprises a cylinder having a v double-acting piston 2| operating therein, and

is secured byvany suitable means, such as bolts 20 22, to one side of the casing 23 of pump I. Piston rod 24 of piston 2| extends through abearing 25 provided at one end of cylinder 20, and the free end thereof is secured to a crosshead 26, A as .shown more particularly in Figs. 1 and 2. 25 Crosshead 28 has secured to the opposite ends thereof the outer ends of a pair of rods 21 and 28. -,These rods extend through suitable bearings. provided in casing 23, and are secured .at. theirinner ends to the sliding blocks I8. The 30 operationof piston 2| is'controlled by a valve mechanism indicated generally by the numeral 29. Valve mechanism 29 ismounted upon a supporting bracket 39 extending laterally from the upper side of cylinder 20, and is secured thereto by bolts 3|, as'shown particularly in Figs. 2 and 3. Valve mechanism 29 comprises a casing 32 having a substantially verticalportion 33, and a substantially horizontal portion 34. The ver--- tical portion -33 of casing 32 has provided therein a .pair of cylindrical chambers ,35 and 35, which are connected at the lower ends thereof by a passage 31, and at a point intermediate the ends thereof by a second passage 39. Chambers 3 35 and 36 have operating therein valves 39 and 40,- respectively. Valve 39, as shown more particularly in Fig. 4, comprises a pair of valve heads 4| and 42 connected-by a reduced portion 43. Extending upwardly from valve head 42 is' a slightly reduced portion which engages an' annularshoulder 45 provided in vertical portion 33 to limit the upward movement thereof. Extending upwardly from reduced portion 44 is a further reduced portion which forms a neck, and extending upwardly from porll 44,. the upper end of tion 46, is a sllghtly enlarged guiding portion 41 which makes sliding contact with an aperture 48 provided at the upper end of portion 33. Fig. 4 shows the position of the parts when in normal position. The left hand end of portion 33 of the casing 32 has provided therein an upper port 49, an intermediate port 50, and a lower port 5|. Ports 50 and 5| are normally closed by plugs 52 and 53 respectively. Valve 40 operating in chamber 36 is similar in construction to valve 39, and also consists of a pair of valve heads 54 and 55 connected by a reduced portion 56. The upper end of valve 40 has a slightly reduced portion 51 which, when valve member 40 is in its upper position, engages an annular shoulder 58 provided near the upper end of portion 33. Valve 40 also has a further reduced portion 59 and a slightly enlarged upper portion 60 extending upwardly therefrom which extends through a guiding aperture 6| provided at the upper end of portion 33. The upper end of casing 33 is provided with a cap member 62, which is secured thereto by any suitable means such as by bolts 63. The right hand end (Fig. 4) of vertical portion 33 has also provided therein an upper port 64, an intermediate port 65, and a lower port 66. Ports 65 and 66 are normally closed by plugs 61 and 68 respectively.

As shown more particularly in Fig. 6, the substantially horizontal portion 3 4 of casing 32 is provided with a transversely extending cylindrical chamber 69, in which operates a valve member 10. Valve member 10 consists of a pair of valve heads 1| and 12, which are connected by means of a reduced portion 13. Portion 34 has secured to the ends thereof a pair of extensions 14 and 15. As shown in Fig. 6, extension 14 is secured to portion 34 by bolts 14', andhas provided therein a cylindrical bore consisting of an enlarged portion 16 and a reduced portion 11.

.The reduced portion 11 has a smaller diameter than chamber 69, and the inner end 18 of extension 14 forms an abutment for valve 10 when the latter has moved to its extreme left hand position. Chamber 11 has operating therein a pisten 19, the reduced portion thereof engaging the left hand end of valve head 1|. A plug 8| closes the left hand end of extension 14.

Extension 15 is of similar construction and is secured to the right hand end of portion 34 by means of bolts 82. Extension 15 is also provided with a central bore consisting of an enlarged portion 83, and a reduced portion 84 in line with chamber 69. A plug 85 closes the outer end of extension 15. The reduced portion 84 has provided therein a piston 86, the reduced portion 81 *of which engages the right hand end of valve head 12. 3

Extensions 14 and 15 have provided at one side thereof ports 88 and 89 which communicate respectively with the enlarged portions 16 and 83 thereof. Portion 34 has also provided therein a pair of spaced ports 90 and 9| communicating with chamber 69. Chamber 69 is also connected intermediate the ends thereof to passage 31 by means of a communicating passage 92.

Vertical portion 33 of casing 32 has also provided at one side thereof a pair of ports 93 and 94 which communicate respectively with chambers 35 and 36 intermediate the ends thereof. As shown in Figs. 2 and 5, when valves 39 and 40 are in their upper position of adjustment, ports 93 and 94 are partially covered by the lower ends of valve heads 42 and 55. Ports 93 and 88 are connected by means of a pipe 95. Ports 94 and 89 are similarly connected by a second pipe 96. Port 90 provided in portion 34 is connected to the inlet port 91 provided at the left hand end of cylinder 20 by means of a pipe 98. Port 9| of portion 34 is likewise connected to the inlet port 99 provided at the right hand end of cylinder 20 by means of a second pipe I00. Port 49 provided at the upper left hand side of vertical portion 33 is connected to the main pipe 3 by a pipe IOI. Port 64 provided at the upper right hand side of vertical portion 33 is likewise connected to main pipe 2 by means of a pipe I02. The lower end of casing 32 is provided with another port I03,

which communicates with passage 31, as shown in Figs. 4, 5 and 6.. This port is connected by means of a pipe I04 to a. small pump I 05 driven by motor I2 through a chain. and sprocket mechanism I06. Fluid is drawn into pump I05 from makeup tank I3 through a pipe I01. Pipe I04 alsohas connected thereto any suitable relief valve I08, which in turn is connected to makeup tank I3 by means of a pipe I09. Relief valve I08 is normally maintained upon its seat by means of a spring I08, the pressure of which may be varied by an adjusting screw I08". It therefore will be observed that by providing an adjustable relief valve of thistype reversing of pump I, and consequently of motors 6 may be efiected at different predetermined pressures.

From the foregoing description it is seen that the upper ends of valve heads 42 and 55 of valves 39 and 40 respectively are constantly connected to the pressure in main pipes 2 and 3 leading from pump I. It is also seen that the lower ends of valve heads 4| and 54 of valves 39 and .40 respectively, are constantly subjected to pressure developed by pump I05. The effective areas of valve heads 4| and 54 are much greater than the effective areas of valve heads 42 and 55, and consequently, valves 39 and 40 will be retained in their uppermost position, shown in Fig. '4, throughout a predetermined operating range. However, in the event the pressure in the main pipes 2 and 3 operating against the upper ends of valve heads 42 and 55 is sufliciently great to overcome the pressure operating at the lower ends of valve heads 4| and 54, valves 39 and 40 will be moved downwardly to uncover ports 93 and 94, as will appear more fully hereinafter.

It will also be observed that when valve 10 is in a, central position, ports 90 and 9| will be covered by valve heads 1| and 12, respectively, and consequently the inner ends of said valve heads will be subjected to equal pressures, and no movement will be imparted thereto. Under these conditions, when the pressure developed by small pump I05 becomes suiilciently great, relief valve I08 will open to permit the discharge of said pump to pass to makeup tank I3.

As previously described, a Passage 38 connects vertical chambers 35 and 36 intermediate the ends thereof. This passage is also connected to a port I I0. which in turn is connected to a vertical passage III, which extends from port IIO to the upper end of vertical portion 33. By this means any leakage of fluid past the guiding portions 41 and 60 of valves 39 and 40 will flow into vertical passage |I|. Port 0 is connected to makeup tank l3 by means of a pipe II2, andconsequently this leakage will drain back to makeup tank I3.

Casing 32 also has provided therein a pair of passages H3 and 4 which connect the opposite ends of chamber 69 of horizontal portion 34 with ports 50 and 65 of vertical portion 33. Thus, 7

any fluid leaking past valve heads II and I2 of valve member I and the pistons I9 and 86 will be forced through these passages and thence into the portion of chambers 35 and 38 disposed between valve heads 4I, 42 and 54,55. Such fluid will then pass through passage 38, port H0, and thence to makeup tank I3 through pipe II2.

Referring to Fig. 2, it will be noted that any fluid from cylinder 20 leaking between piston rod 24 and bearing 25 thereof enters a pocket II5 connected to an outlet port II8 by a communicating passage II1. Port H6 is connected to make-up tank I3 by a pipe 8. Thus, any leakage between piston rod 24 and its bearing 25 will also flow to makeup tank through pocket II5, communicating passage III, and pipe II8'. Mechanism is also provided for varying the stroke of pump I and thus the discharge thereof. Referring to Fig. 1, it will be observed that the left hand sides of sliding blocks I8 have secured thereto the inner ends of a pair of guide rods I and I2I. The'outer ends of these rods are secured to a crosshead I22 having a central aperture through which extends a shaft I23. Shaft I23 is journaled in an abutment plug I24 which makes threaded engagement with the vertical wall I25 of a bracket I28 secured in any suitable manner to the'left hand side of pump I. The inner end of shaft I23 is journaled in a bearing provided in the ,wall I2I of pump casing 23. Shaft I23 has provided thereon a pair of right and left hand threaded portions I28 and I29 which make threaded engagement with a pair of non-rotatable blocks I30 and I3I respectively, mounted in any suitable manner for axial movement toward and from cross-head I22. A hand wheel I32 is secured to the outer end of shaft I23 for rotating the latter. Thus, upon movement of hand wheel I 32 in one direction blocks I30 and I3I .will move away from each other, thereby increasing the distance between said blocks and crosshead I22, and consequently lengthening the stroke of pump I when sliding blocks I8 are adjusted by motor I9. On the other hand, when hand wheel I32 is rotated in the reverse direction blocks I30 and I3I are moved toward each other to decrease the stroke. When blocks I30 and I3I are brought into engagement with crosshead I22 thesliding blocks I8 are centered and consequently the pump is held on neutral stroke. Mechanisms of this type for varying the stroke of a pump are well known in the art, and for a more detailed description thereof reference may be had to United States Patent No. 1,656,034.

From the foregoing description it is thought that the operation of my hydraulic control system is apparent. However, in order to correlate the various elements thereof, a brief rsum of the operation thereof will now be given.

Let us assume hand wheel I32 has been actuated in such a direction as to eifect movement of blocks I30 and I3I away from each other, and valve member I0 has been actuated to the position shown in Fig. 6. Fluid from pump I05 will now enter the right hand end. (Fig. 2) of motor I9 through pipe I04, passages 31 and 92, chamber 89 and pipe I00, causing piston 2|, and consequently sliding blocks I8 of pump I, to move to the left. Pump I will now deliver fluid under pressure to main pipe 2 and suction to main pipe 3. Fluid under pressure will now pass to thev outer ends of cylinders 6 through branch pipes 5, causing the pistons I to move in the feeding direction. The fluid at the other ends of cylinders 8 will be returned to pump I through branch pipes developed by pump I will rapidly increase.

will move downwardly. to uncover port 94.

tank I3 by pipes I36-and I31.

4 and main pipes 3. Pistons I will now continue their movement in the feeding direction until they.

reach their limit of travel whereupon the pressure This pressure is communicated to the upper end of valve 40 through pipe I02, and when the pressure so impressed exceeds that delivered by pump I05, which, it will be remembered, is at all times directed against the lower end of valve 40, the latter When this occurs fluid pressure is delivered against the right hand end (Fig. 6) of piston 86, which in turn actuates valve member I0 to the left. When this occurs communication is established between the left hand end of motor I9 and pump I05, through pipe I04, passages 31, and 92, chamber 69, port 90, and pipe 98 whereby piston 2| moves to the right to reverse the discharge of pump I. When this occurs fluid under pressure will be delivered to pipe 3 and suction to pipe 2, whereupon pistons I operating in cylinders 6 will be actuated in the return direction. When pistons I reach the limit of their travel in this direction pressure again builds up in the system, which pressure is communicated to the upper end "of valve 39. When this pressure exceeds that imposed against the lower end of valve 39 by pump I05 as aforesaid, valve 39 will move downwardly to uncover port 93. The pressure in pipe 3 is now impressed against the left hand end of piston I9, causing valve I0 to be shifted to the right, as shown in Fig. 6. Fluid from pump I05 now passes through tion, said valves are actuated to their upper posi-' tions of adjustment by the pressure imposed at the lower ends thereof by pump I05, thus interrupting the fluid connections between ports 88- 93, and 8994. When valve member I0 is shifted in either direction the fluid at the low pressure side thereof is forced to exhaust pipes 85 or 96, valve chambers and 38, passage 38, port H0,

and pipe -I I2, since it will be remembered that the lower endsof valve heads 42 and 85 only partially cover ports 93 and 94 respectively, when valve members 39 and are in their upper position of adjustment. Any l.eakage past valve heads II and I2 and pistons I9 and 88 will enter passages I I3 or II4, as the case may be, and will be di-- rected to ports 58 or through the portions of chambers 38 and 38 disposed between the valve heads of valve members 39 and 40 operating therein, and intermediate passage 38. Such fluid will then drain back to makeup tank I3 through member I0 for any reason assume a central posi- 1 tion, it may be manually adjusted to either of its effective positions upon removal of plug 8I or 85. To prevent excessive pressures in" the system, especially during reversing periods of the stroke controls of pump I, main pipes 2 and 3 have provided therein relief valves I34 and I35 respectively, which in turn are connected-to makeup Figs. 8 and 9 show the invention applied to a pump I46 wherein the discharge thereof is controlled by a valve I4I. Pump I40 is operated by electric motor I42 through any suitable coupling I48, and receives its fluid from a makeup or supply tank I44 through an intake pipe I45. Pump I40 discharges fluid under "pressure to pipe I46 connected to a port I41 communicating with the central cylindrical chamber I48 of valve I4I. Valve I also has provided therein a pair of spaced ports I49 and I50, to which one end of a pair of pipes II and I52 respectively, are connected. Pipes I5! and I52 are connected by branch pipes I56 and I54, to the opposite ends of the cylinders I55 of a pair of fluid motors I56. Cylinders I55 have pistons I51 operating therein to which feeding rams may be connected in any suitable manner. Valve I4I has also provided therein a port I58 at one end thereof which is connected to makeup tank I44 by means of a pipe I59. The reduced portions I50 and I6I at the opposite ends of chamber I48 are connected by means of a pipe I62. Valve I has provided therein a valve element I69 having a pair of valve heads I64 and I65 connected by a reduced portion I66. Valve element I66 has a stem I61 secured thereto, which extends throughthe end wall I58 of valve MI. The opposite end of valve stem I61 has secured thereto a piston I59 which operates in the cylinder I of a fluid motor I1I secured in any suitable manner, such as by bolts I12 to the casing of valve I. The head end I13 of motor "I is connected to a normally closed port I14 of a control valve I by means of a pipe I16. The other end of motor I1I is connected to a second port I11 of control valve I\15 by means of a second pipe I18. Control valve I15 has provided therein a pair of cylindrical chambers I19 and I80, which are connected at one end thereof by a passage I8I .40 and ports I82 and I88. Chamber I19 has provided therein a valve element I84 consisting of a pair of valve heads I85 and I86 connected by a reduced portion I81. The upper end of valve element I84 has secured thereto a reduced guiding 45 extension I88 which extends through a guiding aperture I89 provided at the upper end of control valve I15. Chamber I80 also has provided therein a similarly constructed valve element I90 consisting of a pair of valve heads I9I and I92 50 connected by a reduced portion I93. One end of valve I90 also has secured thereto a reduced guiding extension I94 which extends through -a second guiding aperture I95 in line with chamber I80. The upper end of chamber I19 is connected "55 to pipe I52 by means of a pipe I96. The upper end of chamber I80 is connected to pipe I5I by a pipe I91. Passage III is connected to a small pump I98 by means of a pipe I99. Pump I96 is connected to shaft 260 of pump I40 by a chain 0 and sprocket mechanism 26I, and fluid is drawn into said pump I98 from makeup tank I44 through a pipe 262. Pipe I99 has provided therein an adjustable relief valve 206 which in turn is connectedto makeup tank I44 by means 05 of a pipe 204. Relief valve 268 may be of the general type shown at I06 in Fig. 1.

v The cylindrical chambers I19 and I86 of control valve I15 are interconnected intermediate the 'ends thereof by means of a transverse passage 70 265. A second passage 205, extending substantially at right angles to passage 205, extends from the latter to the outside of the casing of valve I 15,

and a pipe 261 connects passage 206 with makeup V tank I44. Any fluid leaking past the upper guide portions I66 and I94 of valve-members I94 and I90 respectively, .flows into a vertical passage 208 which extends from the upper end of valve I15 to passage 206. It will be observed that when valve members I84 and I90 are in their uppermost position, the ports I11 and I14 are only partially covered by the valve heads I85 and I9I respectively, Thus, when valve I90 is in the position shown in Fig. 8, and fluid from pipe I5I flows through pipe I91, port I14 and pipe I16 to the head end of piston I69, the fluid at the other end of cylinder I10 will pass through pipe I18, port I11, the annular chamber between valve heads I85 and I85, transverse passage 205, passage 206, and pipe 201, to makeup tank I44. On the other hand, when valve I90 is in its upper position, and valve I84 is in its lower position, fluid from the head end of cylinder I10 will flow to makeup tank I44 through pipe I16, port I14, the chamber between valve heads I9I and I92, transverse passage 205, passage 206, and pipe 201.

From the foregoing description it is seen that port I41 of valve I4I at all times is disposed between valve heads I64 and I65. When the parts are in the position shown in Fig. 8 fluid from pump I40 enters the central chamber I48 of valve I4I through pipe I46 and port I41. Fluid then flows to the right hand ends of cylinders I55 through pipe I5I, causing pistons I51 to move to the left. Fluid at the left hand end'of cylinders I55 then flows to makeup tank through pipes I54,

I52, port I50, chamber I48, the reduced portion I6I thereof, and pipe I59. When pistons I51 reach the limit of their travel in this direction, pressure builds up in pump I40. This pressure is transmitted to the upper end of valve member 1 I90 through pipe I91. When this pressure is suflicient to overcome that imposed against the lower end of said valve member by the small pump I98, valve element I90 moves downwardly, thereby uncovering port I14 to permit fluid under pressure to enter the head end of motor I10. Piston I69 of motor. I1I now actuates valve element I68 to its other extreme position of adjustment, and thereby establishes communication between pump I40 and pipe I52. Fluid now enters the left hand end of cylinders I55, thereby causing the pistons I51 operating therein to move to the right. The fluid at the right hand end of cylinders I55 will now flow back to makeup tank through pipes I53, I5I, port I49, chamber I48, reduced portion I60, pipe I62, reduced portion I6I, and pipe I59. When pistons I51 reach the limit of their travel in this direction, the pressure developed by pump I40 again builds up. Pipe I52, it will be remembered, is connected to the upper end of chamber I19 by means of pipe I96, and when this pressure exceeds that imposed upon the lower end of valve member I 84 by small pump I98, said valve element will move downwardly to uncover port I11. When this occurs, fluid will enter the piston rod end of motor "I through port I11 and pipe I18, causing piston I89 to actuate valve element I56 to the position shown in Fig. 8. Fluid from pump I40 will now again enter the right hand ends. of cylinders I55, causing the pistons I51 operating therein to move tothe left. This procedure is repeated to effect reverse operations of pistons I51 when they reach their limit of traveljin either direction.

It therefore is seen that simplified, as well as highly improved hydraulic control systems have been-provided which positively reverse the discharge of the pump when'the pressure in the system exceeds a predetermined value. In the embodiment shown in Figs. 1 to 7 it will also be noted that the joints between extensions II and I! and the horizontal portion 34 of casing 32 are not subjected to high pressures because 01' passages H3 and H4 which connect the opposite end of chamber 69 with exhaust. The same is true of the joints between the upper end of vertical portion 33 of casing 32 and cap member 62 because of passage III which connects the end or said portion with exhaust. As a result heavy packings are unnecessary to prevent lealrage of fluid from the system. It also is seen that the system is constantly filled with fluid and that any air drawn into the same escapes to exhaust with the leakage through passages H3 or H4 and the'passages communicating therewith, as previously described.

While the embodiments herein shown and described are admirably adapted to fulfill the objects primarily stated, it is to be understood that the invention is not to be limited thereto, since it may be embodied in other forms, all coming within the scope of the claims which follow.

What is claimed is:

1. In a device of the class described, the combination of a reversible discharge pump, motor means, piping connecting said motor to said pump, a fluid motor for actuating said pump to reverse the discharge thereof, an auxiliary pump for actuating said fluid motor, a shiftable valve for controlling the operation of said fluid motor, normally inoperative means operated by the fluid pressure developed in said reversible discharge pump for adjusting said valve, valve means for controlling the operation of said normally inoperative means, means connecting one end of said valve means to the pressure side of said reversible discharge pump, and means connecting the other end of said valve means to the pressure side of said auxiliary pump, whereby when the pressure imposed upon said valve means by said reversible discharge pump exceeds that imposed thereon by said auxiliary pump, said valve means operates to render said normally inoperative means operative to eilfect adjustment of said shiftable valve.-

2. In a device of the class described, the combination of a reversible discharge pump, motor means, piping connecting said motor means to said pump, a fluid motor for actuating said pump to reverse the discharge thereof, an auxiliary pump for actuating said fluid motor, a shiftable valve for controlling the flow of fluid from said auxiliary pump to said fluid motor, normally inoperative servomotor means operated by fluid pressure developed by the reversible discharge pump for actuating said shiftable valve to difierent positions of adjustment, a valve mechanism comprising a pair of valve elements for controlling the operation of said servomotor means,

means connecting one end of one of said valve elements to the pressure side of said reversible discharge pump when the latter is discharging fluid in one direction, means connecting one end of the other of said valve elements to the pressure side of said reversible discharge pump when the latter is discharging fluid in the reverse direction, and means connecting the opposite end of each of said valve elements to the pressure side of said auxiliary pump, whereby when the pressure imposed upon either of said valve elements by said reversible discharge pump exceeds that imposed thereon by said auxiliary pump the said valve element subjected to such pressure operates to render said servomotor means operative to effect adjustment of said shiftable valve.

3. In a device of the class described, the combination of a main pump a double-acting fluid motor operated by the said main pump, means controlling the flow of fluid from said pump to said fluid motor, a servomotor for actuating said controlling means,an auxiliary pump for actuating said servomotor, a shiftable valve for controlling the operation of said servomotor, normally inoperative means operable by the fluid pressure developed in said main pump for adjusting said shiftable valve, valve means for controlling the operation of said normally inoperative means, means connecting one end of said valve means to the pressure side of said main pump, and means connecting the other end of said valve means to the pressure side of said auxiliary pump, whereby when the pressure imposed upon said valve means by said main pump exceeds that imposed thereon by said auxiliary pump said valve means operates to render said normally inoperative means operative to effect adjustment of said shiftable valve.

4. In 'a device of the class described, the combination of a mainpump, a double-acting fluid motor operated by the said main pump, means controlling the flow oifluid from said pump to said fluid motor, a servomotor for actuating said controlling means, an auxiliary pump for actuating said servomotor, a shiftable valve for con- ,trolling the operation of said servomotor, normally inoperative meansoperable by the fluid pressure developed in said main pump for adjusting said shiftable valve, valve means for controlling the operation of said normally inoperative means, means connecting one end of said valve means to the pressure side of said main pump, means connecting the other end'of said valve means to the pressure side of said auxiliary pump, whereby when the pressure imposed upon said valve means by said main pump exceeds that imposed thereon by said auxiliarypump said valve means operates to render said normally inoperative means operative to efiect adjustthe operation of said servomotor, motor means operable by the fluid pressure developed in said main pump' for adjusting said shiftable valve,

' valve means for controlling the operation of said motor means, means connecting one end of said valve means to the pressure side of said main pump, and means connecting the other side of said valve means to the pressure side of said auxiliary pump, whereby when the pressure imposed upon said valve means by said main pump exceeds that imposed thereon bysaid auxiliary pump, said valve means operates to connect said motor means for operation to eiiect adjustment of said shiftable valve.

6. In a device of the class described, the combination of a main pump, a double-acting fluid motor operated by said pump, means controlling the flow of fluid from said main pump to said fluid motor, a servomotor for actuating said controlling means, an auxiliary pump for actuating said servomotor, a shiftable valve for controlling the flow of fluid from said auxiliary pump to said servomotor, motor means operable by the fluid pressure developed by said main pump for actuating said shiftable valve to different positions of adjustment, a valve mechanism comprising a pair of valve elements for controlling the operation of said motor means, means for alternately connecting one end of each of said elements to the pressure side 01' said main pump, and means connecting the opposite end of each of said elements to the pressure side of said auxiliary pump, whereby when the pressure imposed upon either of said valve elements by said main pump exceeds that imposed thereon by said auxiliary pump, the said valve element subjected to such pressure operates to render said motor means operative to effect adjustment of said shii'table valve.

7. In a device of the class described, the combination of a main pump, a double-acting fluid motor operated by said pump, means controlling the flow of fluid from said main pump to said fluid motor, a servomotor for actuating said controlling means, an auxiliary pump for actuating said servomotor, a shittable valve for controlling the flow of fluid from said auxiliary pump to said servomotor, motor means operable by the fluid pressure developed by said main pump for actuating said shiftable valve to diflerent positions of adjustment, a valve mechanism comprising a pair of valve elements for controlling the operation of said motor means, means for alternately connecting one end of each of said elements to the pressure side of said main pump, means connecting the opposite end of each of said elements to the pressure side of said auxiliary pump, whereby when the pressure imposed upon either of said valve elements by said main pump exceeds that imposed thereon by said auxiliary pump, the said valve element subjected to such pressure operates to render said motor means operative to eflect adjustment 01' said shiftable valve, and means for varying the pres- 

